Dual-mode conical horn antenna



Dec. 2, 1969 E. R. NAGELBERG 3,482,252

DUAL-MODE CONICAL HORN ANTENNA Filed Nov. 29. 1966 FIG.

(PRlOR ART) SOURCE (C -C AS FUNCTION OF FREQUENCY 0 A.SMALL D|AMETER=2.|o B, LARGE DIAMETER=2.8 u no 1 I l 1 5.| 5.3 5.5 5.7 5.9 6.l 6.3 6.5 6.76.9

FREQUENCY IN KILOMEGACYCLES/ SEC.

m/ ws/v TOR E. R. NAGELBERG A TTORNE Y United States Patent O US. Cl.343-786 5 Claims ABSTRACT OF THE DISCLOSURE A horn antenna wherein thehorn member is excited by more than one electromagnetic mode signal. Theuse of a predetermined configuration for the horn member mitigates theeffect of aperture phase dispersion.

BACKGROUND OF THE INVENTION Field of the invention This inventionpertains to antennas and, more particularly, to dual-mode conical hornantennas.

Description of the prior art The dual-mode conical horn antenna, forexample of the type described in an article by P. D. Potter, entitled ANew Horn Antenna with Suppressed Sidelobes and Equal Beamwidths, in theJune 1963 issue of the Microwave Journal, page 71, has proven to be avery useful primary feed element for low noise antennas, particularlythose of the Cassegrain type. Its applicability to other general uses isnow recognized as one of its more important features. By exciting theantenna horn aperture with an appropriate combination of spherical TE(dominant) and TM modes it is possible to produce a horn radiationpattern with approximately equal E-plane and H-plane beam widths, andsidelobe levels substantially lower than those for conventionallyexcited (TE only) horns with the same aperture dimensions. Thisimprovement, obtained at a modest expenditure of gain, is achieved byequalizing the E-plane and H-plane illumination taper, thereby reducingthe E-plane edge current and the corresponding sidelobe levels.

Generally, the most important phenomenon affecting dual-mode hornperformance, over a band of frequencies, is aperture phase dispersion, aterm used to describe the difference in relative phase between the TEand TM modes at the horn opening. For optimum performance it is requiredthat the radial components of the electric field for the two modes be180 degrees out of phase at the edge, i.e., at the boundary walls of theantenna.

Potter, in the above-mentioned article, teaches that mode conversionfrom the TE mode to the TM mode may be effected by a simple steptransition between guide sections of a first and second predetermineddiameter. The length of the guide section of said second predetermineddiameter, identified by Potter as the phasing section, is selected toaccount for the relative phase lengths of the horn for the twopropagating modes in the horn. Since overall antenna specificationsusually determine the antenna aperture diameter, cone angle, anddiameter of the phasing section, the length of the horn section istherefore determined, and, thus, so is the relative phase shift betweenthe two propagating modes. Correction of this difierential phase shiftat the horn aperture by varying the length of the phasing section, asper Potter, quite often results in a deterioration of overall antennaperformance, rather than an improvement in performance. As the length ofthe phasing section is increased, its inherent phase dispersion withchanges in operating fre- 3,482,252 Patented Dec. 2, 1969 ICC quencyalso increases, causing a degradation of the circular symmetry of theantenna radiation pattern, a decrease in overall antenna gain and anincrease in unwanted sidelobe levels.

SUMMARY OF THE INVENTION It is, therefore, an object of this inventionto mitigate the etfects of aperture phase dispersion in order tooptimize the performance of dual-mode conical horn antennas.

It is another object of this invention to mitigate the elfect ofaperture phase dispersion without resorting to lengthy and highlydispersive phasing sections.

In accordance with this invention, the effect of aperture phasedispersion is mitigated by the use of a horn antenna of a predeterminedconfiguration. More particularly, a dual-mode conical horn is utilizedwhich is characterized by a double flare. One section of the horn, i.e.,the frustrum of a cone, has a predetermined half angle to give thecorrect aperture dimensions, and another section, i.e., a secondfrustrum of a cone, has a half angle selected to yield the correctrelative phase. Thus, the dispersive phasing section, relied upon in theprior art, is relegated to a vernier or trimming function, required onlyto compensate for small errors in design and construction. The necessityfor a lengthy and highly dispersive phasing section is thereforeeliminated with a corresponding improvement in antenna performance.

These and further features and objects of this invention, its nature andvarious advantages may be more readily apprehended and understood uponconsideration of the attached drawings and of the following detaileddescription of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an illustrative diagram of aprior art dualmode conical horn antenna;

FIG. 2 is an illustrative diagram of a dual-mode conical horn antennawhich embodies the principles of this lnvention; and

FIG. 3 is a graphical presentation of the difi'erential phase shift of aparticular waveguide configuration.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION In order to appreciate theinvention more fully, FIG. 1 is included to represent a typical priorart dual-mode conical horn antenna.

Source 11, of FIG. 1, excites in hollow waveguide section 12, ofcircular cross section, energy which propagates in the TE (dominant)wave mode. The diameter A of section 12 is selected, in a manner wellknown to those skilled in the art, so that the guide is cut off formodes other than the TE The step discontinuity 13, which forms aconductive boundary between guide section 12 and hollow waveguidesection 14, also of circular cross section, converts a portion of theincident TE energy to the TM Wave mode. Mode conversion via the use of adiscontinuity is discussed in detail in an article by J. Shefer and meinthe Bell System Technical Journal, vol. 44, at page 1321, entitledMode Conversion in Circular Waveguides. The combined TE -TM wave modemixture propagates in horn section 15, e.g., a hollow frustrum of acone, and is radiated into the atmosphere at aperture 16.

As discussed above, the principal difficulties in the design ofdual-mode horn antennas are in properly phasing the two modes, TE and TMand maintaining the correct relative phase over the frequency range ofinterest. Generally, design specifications comprise the horn aperturediameter, indicated as C in FIG. 1, the half angle flare 0 of theconical horn, and the relative power in the two propagating modes, thespecifications being determined on the basis of overall antennarequirements. Usually, there is a certain degree of freedom in selectingthe diameter B of guide section 14; it should be made sufficiently largeso that the TM mode is well above cutoff, but not so large that therequired input waveguide 12, itself, becomes overmoded. A detaileddiscussion of antenna requirements and their relationship to antennadimensions may be found in the article entitled The Open CassegrainAntenna: Part I, Electromagnetic Design and Analysis, authored by J. S.Cook, E. M. Elam and H. Zucker, appearing in vol. 44 of the Bell SystemTechnical Journal, at page 1255.

The most important phenomenon affecting dual-mode performance isaperture phase dispersion, a term used to describe the difference inrelative phase between the TE and TM modes at the horn aperture 16.Proper phasing of these two modes at the horn aperture is therefore thedominant problem in constructing an efficient dual-mode conical hornantenna. Unfortunately, once the aperture diameter C, cone angle andguide diameter B are specified, the length of the horn section 15 isalso determined and so, therefore, is the relative phase shift betweenthe two modes. Quite often, the error in relative phase between the twomodes at the aperture is so large that it becomes necessary to use along and therefore highly dispersive phasing section 14. For example, ina particular horn antenna it was found that in order to substantiallydecrease the relative phase shift between the two modes, a phasingsection 14 of length L was required which had phase dispersion byitself, over a ten percent frequency band, equal to approximately 25degrees. Coupled with the intrinsic phase dispersion of the horn 15,which may, by itself, be of the same order of magnitude, the totalaperture phase dispersion would be approximately 50 degrees. The effectof such a large phase dispersion, especially in a reflector feed system,is to decrease the circular symmetry of the radiation pattern and alsoto cause a decrease in the antenna gain with a concomitant increase inunwanted sidelobe levels.

This deleterious effect of prior art antennas may be eliminated inaccordance with the principles of this invention by using a horn antennaof the type illustrated in FIG. 2. The antenna of FIG. 2 ischaracterized by a horn with a double flare, one section 1512, to yieldthe correct aperture dimensions (diameter and angle) and another section15a, Whose angle is selected to yield the correct relative phase at theaperture 16. The phasing section 14, is thus relegated to a trimmingfunction used only to compensate for small errors in design andconstruction.

More particularly, analogous to the operation of the antenna of FIG. 1,source 11 of FIG. 2, excites in waveguide section 12 a propagating wavein the TE mode. The step discontinuity 13, which forms a conductiveboundary between guide sections 12 and 14, converts a portion of theincident propagating energy into the TM mode. The combined wave modes,TE and TM propagate in horn sections 15a and 15b and are radiated intothe atmosphere at aperture 16.

The horn of the antenna, as illustrated in FIG. 2, comprises twofrustrums, 15a and 15b, each having a predetermined half angle 6 and 0respectively. As discussed above, general antenna requirements determinethe magnitude of the horn aperture diameter C, the angle 0 of section15b, and the relative diameters A and B of guide sections 12 and 14. Inorder to substantially reduce the eifect of phase aperture distortion,section 15a of the horn, in accordance with the principles of thisinvention, is constructed to have a different half angle 0 than that ofsection 15b. Generally, the value of the half angle 6 is determined tobe such that the relative phase between the TE and TM modes at the hornaperture is equal to an odd multiple of 11' radians. This relative phasecan be calculated from the relationship {ULTIVF OTE) aperture(ITEM-(1T2?) disc.

+(TMTE)U TM- Tn)e where n=integer, and (a u in each case denotes therelative phase shift between the two modes in the respectivelyidentified sections. Thus, (a a denotes the relative phase at the hornaperture, (m a denotes the relative phase at the discontinuity betweendiameters A and B, (oL a denotes the ditferential phase shift in thefrustrurn of half angle 0 and (oc a denotes the dilferential phase shiftin the frustrum of half angle 0 The quantity h is calculated by solvingthe electromagnetic boundary value problem of the given discontinuityimpinged upon by electromagnetic energy in the form of a TE mode. Forexample, FIG. 3 shows as a function of frequency for A=2.1 inches andB=2i8 inches.

The quantities L and (a a can be expressed, respectively, in terms of 0or 0 and the diameters B, C and D, from the well-known formulasdescribing the propagation of electromagnetic waves in conicalwaveguides. See, for example, F. Borgnis and C. H. Papas,Electromagnetic Waveguides and Resonators, Encyclopedia of Physics, vol.XVI, Springer, Berlin, 1958, page 356.

In a specific application, a horn antenna for use at X-band frequencieswas constructed in accordance with the principles of this inventionhaving the following dimensrons:

A=7.2/k, B=9.6/k, C=18.8/k, D=16.8/k, 0 =5.5 and 0 :67, where k is equalto 21r divided by the signal wavelength at the center frequency of theoperating band. The antenna was provided with a variable trimmingsection 14, whose length could be adjusted in order to optimize theperformance of the antenna. Phasing of the two propagating modes wasachieved by monitoring the longitudinal wall currents and varying thelength of section 14 until a minimum was observed. This condition occurswhen the electric fields of the two modes perpendicular to the walls ofthe horn are the required 180 degrees out of phase. The length L ofsection 14 for this condition was approximately .26)\, where A is thesignal wavelength at the center frequency of operating band,corresponding to approximately one-tenth the length required if priorart techniques had been utilized. Thus, proper phasing of the two modesis accomplished without resorting to long phasing sections which arehighly dispersive and degrade overall antenna performance.

It is to be understood that the embodiments shown and described areillustrative of the principles of this invention only, and that furthermodifications of this invention may be implemented by those skilled inthe art without departing from the scope and spirit of the invention.For example, though the principles of this invention are particularlyapplicable to feed horn antennas, they are also applicable to any othersituation Where horn antennas are used. In addition, diverseconfigurations may be implemented using mode signals other than the TEand TM I claim:

1. A dual-mode conical horn antenna comprising:

a first circular hollow waveguide section having a first predetermineddiameter,

a second circular hollow waveguide section having a second predetermineddiameter conductively coupled to said first waveguide section,

a third truncated conical hollow waveguide section having a firstpredetermined half angle for reducing the phase differential betweenmode signals propagating in said third waveguide section conductivelycoupled to said second waveguide section, and

a fourth truncated conical hollow waveguide section having a secondpredetermined half angle conductively coupled to said third waveguidesection.

2. An improved dual-mode conical horn antenna,

4. An improved dual-mode conical horn antenna comwherein mode conversionis accomplished by a discontinuity between first and second hollowwaveguide sections, having a horn member comprising:

a third hollow waveguide section comprising a frustrum of a cone havinga first predetermined half angle for diminishing the aperture phasedispersion of propagating mode signals, and

a fourth hollow waveguide section comprising a frustrur'n of a conehaving a second predetermined half percentage of said mode signal into asecond mode "signal,

a horn member,

a first section of said horn member, responsive to said first and secondmode signals, comprising a frustrum of a cone' having a firstpredetermined angle for diangle, 10 minishing'the aperture phasedispersion between said wherein said third and fourth waveguide sectionsare two inode signals, and a second section of said horn conduetivelyunited and have a common axis of member comprising a frustrum of a conehaving a propagation. second predetermined angle for radiating saidsignals 3. A dual-mode conical horn antenna comprising: at the aperturethereof.

a first circular hollow conduetively bounded waveguide 5. An antennahaving a horn member responsive to section of first predetermineddiameter having input and output means, said input means responsive toapplied electromagnetic signals,

a second circular hollow conductively bounded wavemore than onemicrowave mode signal wherein said horn member comprises:

a first hollow conductive frustrum of a cone having a first preselectedhalf angle for reducing the phase guide section of second predetermineddiameter having input and output means,

conductive means uniting said first waveguide section output means andsaid second waveguide section indifferential of said mode signals, and

a second hollow conductive frustrum of a cone having a secondpreselected half angle for radiating said mode signals.

put means for converting a proportional part of incident electromagneticenergy into a different order mode signal,

a third hollow conduetively bounded waveguide sec- References CitedUNITED STATES PATENTS tion comprising a frustrum of a cone having afirst 3,373,431 3/1968 Webb 343 786 predetermined half angle coupled tosaid second FOREIGN PATENTS waveguide section output means for reducingthe phase differential between propagating mode signal 9 05 5/1957 Germay,

and

a fourth hollow conduetively bounded waveguide section comprising afrustrum of a cone having a second predetermined half angle coupled tosaid third waveguide section.

H. K. SAALBACH, Primary Examiner T. VEZEAU, Assistant Examiner

